A terrace of garden paving slabs lasts mainly because of what lies beneath the slabs: a stable, free-draining, well-compacted sub-base with a slight fall to carry water away. The choice of material (concrete, natural stone or porcelain stoneware) and of laying method (gravel bed, adjustable pedestals or bonding onto a concrete slab) depends on your ground, your climate and how you plan to use the space. In French-speaking Switzerland, frost resistance and the slipperiness of wet surfaces matter as much as appearance.
Choosing your slabs: concrete, natural stone or porcelain stoneware
The three families of slabs differ in appearance and in how they behave outdoors. Before choosing, check that the product is sold as suitable for outdoor use and frost-resistant: an indoor slab cracks or flakes after a few winters.
- Concrete slabs: a wide choice of sizes, colours and finishes, and simple to lay. Very smooth surfaces can be slippery when wet, and some colours age faster than others.
- Natural stone (granite, sandstone, limestone, slate, etc.): a unique look and good durability, but each stone behaves differently. Some are porous and soak up water, making them vulnerable to frost, or stain easily. Ask for the technical data sheet of the specific stone you are considering.
- Porcelain stoneware: a very dense, barely porous material that withstands frost well and is easy to clean. It is usually laid in a thin section, which calls for a very even base. Choose a non-slip finish for outdoor use.
Think about colour too: a dark slab heats up strongly in the sun, while a very pale one dazzles and shows dirt more.
The three laying methods and their limits
| Laying method | Advantages | Limits |
|---|---|---|
| On a bed of gravel or sand | Free-draining, relatively simple, slabs can be lifted | Needs a carefully prepared sub-base; sinking appears if the base is poorly compacted; joints get dirty |
| On adjustable pedestals | Quick levelling, water drains away underneath, useful on a flat roof, over an existing slab or to compensate for a slope | Slabs of good thickness needed; the void under the terrace must be protected from pests and leaves; height to check |
| Bonded onto a concrete slab | Very stable, uniform surface, fine joints | Heavier works, concrete base to be built with fall and drainage, hard to undo; adhesive and grout must be suitable for frost |
There is no ideal method in itself. On well-drained, level ground, a gravel bed is often suitable. On damp or sloping ground, or over an old slab, pedestals or a concrete slab become more relevant.
The technical points that make a terrace last
Most problems (slabs that move, puddles, failing joints) come from something overlooked at the start.
- The sub-base. Remove the topsoil, then build up a layer of free-draining material, spread in layers and compacted. A geotextile between the soil and the gravel limits the mixing of materials. On clay soil or soil that holds water, the sub-base needs more careful planning.
- The fall. The surface must slope slightly, around 1 to 2%, carrying water away from the house. A perfectly flat terrace holds water, encourages moss and, in winter, ice.
- The joints. Depending on the method, they are filled with sand, fine gravel or jointing mortar. An open joint lets water through but lets weeds in; a closed joint is cleaner but must remain compatible with frost and expansion.
- Frost. Water that seeps in and then freezes lifts and cracks the surface. Good drainage beneath the slabs, frost-proof materials and a sufficient fall are essential, all the more so at altitude and in the Jura uplands, where freeze-thaw cycles are frequent.
- Slipperiness. A wet, shaded or moss-covered surface quickly becomes dangerous, especially near a pool, a staircase or a slope. Prefer a textured or non-slip finish.
- The edging. It holds the slabs in place around the perimeter, stops the bedding layer from escaping and marks the boundary with the rest of the garden. It is laid before the slabs, on a stable foundation.
- The junction with the lawn and borders. Leave the terrace slightly above ground level or flush with it, with an edging or a strip of gravel, to make mowing easier and keep soil out of the joints. If you want to add a covered area, the pergola or veranda should be planned at the same time as the terrace, as its supports affect the base.
Care: cleaning, moss and replacing a slab
A slab terrace needs little maintenance, but not none.
- Routine cleaning: regular sweeping, then washing with water and a mild product. Check that the product suits your material, as some stones cope badly with acids.
- Moss and algae: they settle first in the shade and where water stagnates. Cleaning in spring and autumn, better drainage and a little pruning nearby limit the problem. Pressure washing should be used with care: too strong, and it damages the surface and empties the joints.
- Joints: to be refilled when they hollow out or no longer resist weeds well.
- Replacing a broken or sunken slab: remove the slab, check and re-level the bedding layer, then lay an identical slab. Keep a few spare slabs from the same batch, as colours vary from one production run to another. If several slabs sink in the same place, it is the sub-base that needs redoing, not just the slabs.
By region: lakeside, plain, mountains
Climate changes the way a terrace should be designed in French-speaking Switzerland.
- Lake Geneva and other lakeshores: winters are milder and humidity is higher. In Geneva or Montreux, moss and slipperiness are often the main issue, along with sun exposure on plots sloping towards the lake.
- Plateau and plain: temperature swings are more marked, with frost in winter and sometimes heavy or clay soils. In Neuchâtel, as on the Plateau in general, the drainage of the sub-base deserves particular care.
- Valais: strong sunshine and large temperature swings between day and night, which stress the joints and heat dark slabs. At altitude, frost adds to this.
- Jura uplands and Pre-Alps: snow, deep frost and many freeze-thaw cycles. You need clearly frost-resistant slabs, a distinct fall and a highly draining base, and you should plan for snow clearing without damaging the surface.
Doing it yourself or hiring a professional
Laying a few slabs on a gravel bed, on level, sound ground and for a small area, remains within reach of a methodical DIYer: most of the work is excavation, compaction and levelling. The work is physical, and a levelling error at the start shows across the whole surface.
A professional is needed for a large area, sloping ground, soil that holds water, bonding onto a concrete slab or a raised terrace. If the terrace is raised, close to a property boundary or linked to a wall, check with your commune before starting: rules on distance and height, or a permit, may apply. To compare the options, see the page on terrace decking or stone and on landscape design for the whole garden.
SOS Jardin is a matching service: you describe your project, and your request is passed to a professional gardener and landscaper in your region, who contacts you and prepares a free, no-obligation quote. You can request a quote in a few minutes.
Frequently asked questions
What is the best slab for a garden terrace?
There is no single best option. Porcelain stoneware resists frost well and is easy to clean, natural stone offers a lasting look but varies with the rock, and concrete offers a wide choice and is simple to lay. Always check that the slab is intended for outdoor use and is frost-resistant.
Do you need a concrete slab under a paved terrace?
Not necessarily. On sound, well-drained ground, a compacted sub-base with a gravel bed is often enough. A concrete slab becomes useful for bonded laying, for unstable ground or when you want a very even surface, provided it is given a fall and drainage.
How do you stop slabs from moving or sinking?
The sub-base is what matters most: topsoil removed, free-draining material compacted in layers, careful levelling. Solid edging around the perimeter and good water drainage complete the job.
How do you stop moss from settling on the terrace?
Moss appears in shaded, damp spots. Improving water drainage, pruning plants that cast shade and cleaning the surface regularly reduce the problem. A low-porosity material such as porcelain stoneware is easier to clean.
Do you need a permit to build a terrace?
It depends on the commune and the project. A ground-level terrace is often simpler than a raised one or one built on a property boundary. Check with your commune’s building department before starting work.